# High Eosinophils in Cats: What Eos on Bloodwork Means Eosinophils (often shortened to EOS on cat bloodwork) are a type of white blood cell that helps fight parasites and plays a role in allergic reactions [1]. When your cat's EOS value is high, it usually points toward parasites, allergies, or less often a bone marrow or immune condition. When it is low, it is rarely meaningful on its own. This article is educational and is not a substitute for veterinary diagnosis or treatment. ## Quick Answer - Eosinophils are normal white blood cells. They are not automatically a sign of cancer or a crisis. - A high eosinophil count in cats is most often linked to parasites (especially intestinal worms and heartworm) or allergic disease such as flea allergy, food allergy, or asthma [1]. - Mild eosinophilia is common and often incidental. In one referral hospital population, about 9.3% of cats had eosinophilia on routine blood counts [2]. - A low eosinophil count is usually not concerning by itself and often reflects stress, illness, or steroid medications. - The number alone does not give a diagnosis. Your veterinarian combines it with your cat's history, physical exam, and targeted tests. ## What Eosinophils Look Like on Cat Bloodwork ### What the EOS value actually is A complete blood count (CBC) measures the different types of white blood cells. Eosinophils are one of the granulocytes, a group that also includes neutrophils and basophils [1]. Neutrophils handle most bacterial infections, while eosinophils focus on parasites and allergic responses [1]. On a lab report, you will usually see "EOS" or "eosinophils" listed with a number and a reference range. The range varies by laboratory, so always compare your cat's value to the range printed on that specific report. ### What counts as high in a cat Different labs and studies use different cutoffs. One large study defined eosinophilia in cats as 950 cells/μL or higher [2]. Other references use a lower threshold. This is why your veterinarian interprets the number in context instead of applying one universal cutoff. ### What you might notice at home Many cats with a high eosinophil count look completely normal. The count is often found by accident during routine bloodwork. When there are signs, they tend to reflect the underlying cause, such as: - Itchy skin, scabs, or hair loss - Vomiting or diarrhea - Coughing or wheezing - Weight loss or poor appetite - Lethargy The eosinophil number itself does not cause symptoms. The disease driving it does. ## Main Causes of High Eosinophils in Cats Eosinophilia has a wide range of possible causes. Your veterinarian will weigh the size of the increase, your cat's age, lifestyle, and symptoms. | Cause | Typical clues | How urgent | |---|---|---| | Intestinal parasites (roundworms, hookworms, tapeworms) | Young cat, outdoor access, diarrhea, potbelly, poor coat | Routine visit, treat promptly | | Heartworm disease | Cough, exercise intolerance, indoor or outdoor cat in endemic area | Same-day to urgent visit | | Flea allergy dermatitis | Intense itching, scabs near the tail base, visible fleas or flea dirt | Routine visit | | Food or environmental allergy | Chronic itching, head and neck lesions, vomiting, diarrhea | Routine visit | | Feline asthma | Coughing, wheezing, open-mouth breathing | Urgent if breathing is labored | | Eosinophilic granuloma complex | Raised sores on lips, skin, or mouth | Routine visit | | Fungal or other infections | Nasal discharge, sneezing, skin nodules | Same-day visit | | Hypereosinophilic syndrome | Persistent high count, weight loss, organ involvement | Urgent, specialist care often needed | | Paraneoplastic eosinophilia (linked to some cancers) | Weight loss, vomiting, enlarged lymph nodes | Urgent, needs imaging and biopsy | | Chronic eosinophilic leukemia | Very high count, immature eosinophils on smear | Urgent, referral recommended | Parasites are the classic trigger. A cross-sectional study at a veterinary teaching hospital looked at whether eosinophilic cats actually had intestinal or respiratory parasites, and the authors noted that the link between eosinophilia and parasite infection is commonly assumed but the evidence is limited [2]. That does not mean parasites are rare. It means your veterinarian will confirm the cause with testing instead of guessing. Allergic disease is another major category. Eosinophils are involved in allergic reactions, so conditions like flea allergy, food allergy, and asthma often raise the count [1]. If your cat has chronic skin or airway problems, see our guides on [eosinophilic granuloma in cats](/knowledge/veterinary-medicine/preventive-care/eosinophilic-granuloma-in-cats) and [cat asthma](/knowledge/veterinary-medicine/symptom-guides/cat-asthma). Less common causes include hypereosinophilic syndrome, a rare disorder where eosinophils build up and damage tissues [3]. One case report described a young cat with sustained eosinophilia who developed heart failure from eosinophilic damage to the heart valves and muscle [3]. Another report described a cat with intestinal lymphoma and a very high eosinophil count, a paraneoplastic effect where the tumor drives eosinophil production [4]. A third report described suspected chronic eosinophilic leukemia in an older cat with progressive eosinophilia and immature eosinophils on the blood smear [5]. These severe causes are uncommon, but they are why a persistently high count deserves follow-up instead of watchful waiting. ## What Low Eosinophils Mean A low eosinophil count is called eosinopenia. It is far less useful than a high count. Common reasons include: - Stress or illness - Corticosteroid medications (prednisolone, dexamethasone) - Acute infection or inflammation - Recent vaccination In most cases, a low EOS value on its own does not require treatment. Your veterinarian will look at the rest of the CBC and your cat's clinical picture. For a deeper look at how the whole white cell pattern fits together, see [leukogram patterns in dogs and cats](/knowledge/veterinary-medicine/clinical-pathology/leukogram-patterns-dogs-cats). ## Red Flags That Need Emergency Care Call an emergency clinic right away if your cat has: - Open-mouth breathing, blue gums, or severe respiratory distress - Collapse, weakness, or inability to stand - A distended abdomen with labored breathing - Persistent vomiting with no eating or drinking for more than 24 hours - High fever with lethargy - Sudden swelling of the face, throat, or airway Breathing problems in a cat are always urgent. Cats hide respiratory disease well, and by the time they breathe with an open mouth, they are often in serious trouble. ## Safe Things You Can Do at Home While Waiting You cannot lower your cat's eosinophil count at home. What you can do is gather information and avoid making things worse. - Do not give any human medication. Many common drugs, including acetaminophen and ibuprofen, are toxic to cats. - Do not start or stop steroid medications without your veterinarian. Steroids change the eosinophil count and can mask the underlying disease. - Write down a symptom timeline. Note when itching, vomiting, coughing, or appetite changes started. - Photograph skin lesions, stool changes, or anything unusual. Bring the photos to your visit. - Keep your cat indoors and away from unknown cats until you have a diagnosis. - Collect a fresh stool sample if possible. Your veterinarian may want it for parasite testing. - Use flea prevention that is labeled for cats and prescribed or approved by your veterinarian. If your cat is stable and eating, a routine appointment is reasonable. If your cat is not eating, breathing hard, or acting painful, do not wait. ## What Your Veterinarian Will Check and Test ### History and physical exam Your veterinarian will ask about indoor and outdoor access, flea prevention, diet, travel history, other cats in the home, and any past steroid use. The physical exam looks for skin lesions, enlarged lymph nodes, abdominal discomfort, heart murmur, and breathing changes. ### Repeat bloodwork A single high value may be a fluke or a lab variation. Repeating the CBC helps confirm whether the eosinophilia is persistent. Persistent or rising counts need more investigation. ### Fecal testing Fecal flotation and Baermann testing look for intestinal and respiratory parasites [2]. These tests are simple and often the first step. ### Heartworm testing Heartworm disease is a known cause of eosinophilia in cats. Testing is especially important in endemic regions. ### Allergy testing If parasites are ruled out and your cat has skin or airway signs, allergy testing may be considered. For cats with respiratory allergy, measuring specific IgE to the major cat allergen Fel d 1 is one diagnostic approach used in human medicine, and positive results have been linked to higher total IgE and eosinophil counts [6]. In cats, allergy testing is typically reserved for specific situations and interpreted alongside the clinical exam. ### Imaging X-rays and ultrasound can reveal asthma patterns, organ enlargement, masses, or fluid. These are common next steps when the count is high and symptoms are present. ### Bone marrow sampling and biopsy If your veterinarian suspects a bone marrow disorder, leukemia, or cancer, they may recommend a bone marrow sample or tissue biopsy [5][4]. These are usually done with a specialist. ### Referral Cats with sustained, marked eosinophilia or signs of organ damage may be referred to an internal medicine specialist. Hypereosinophilic syndrome and related conditions are rare and complex [3]. See our article on [feline hypereosinophilic syndrome](/knowledge/veterinary-medicine/clinical-methods/feline-hypereosinophilic-syndrome-diagnosis-management) for more detail. ## Prevention and Monitoring You cannot prevent every cause of eosinophilia, but you can reduce the most common triggers. - Keep parasite prevention current year-round. Follow your veterinarian's product and schedule. - Control fleas rigorously. Flea allergy is a common driver of eosinophilic skin disease. - Feed a consistent, complete diet. If a food allergy is suspected, your veterinarian will guide an elimination trial. - Keep cats with asthma on their prescribed plan and recheck as directed. - Schedule routine wellness bloodwork, especially for older cats or cats with chronic conditions. - Track trends over time. A single number matters less than whether it is rising, falling, or stable. If your cat has a known eosinophilic condition, your veterinarian will set a monitoring schedule based on the diagnosis, response to treatment, and your cat's overall health. ## Limitations and When to Contact a Veterinarian A high eosinophil count is a clue, not a diagnosis. Many cats with mild eosinophilia turn out to have a mild, treatable problem. Some have something serious. You cannot tell which from the number alone. Contact your veterinarian the same day if your cat has new vomiting, diarrhea, poor appetite, or skin lesions that are spreading. Go to an emergency clinic now if your cat is breathing with an open mouth, collapsing, or cannot be roused. Schedule a routine visit if the only finding is an abnormal EOS value on otherwise normal bloodwork and your cat seems well. Every case is different. Your veterinarian needs to examine your cat and may need repeat testing to give you a real answer. ## Frequently Asked Questions ### Does a high eosinophil count mean my cat has cancer? No, most cats with high eosinophils do not have cancer. Parasites and allergies are far more common causes. Cancer and bone marrow disorders are possible but uncommon, and they usually come with other abnormal bloodwork or symptoms. ### Can allergies alone cause high eosinophils in cats? Yes. Eosinophils are part of the allergic response, so flea allergy, food allergy, and environmental allergy can all raise the count [1]. If your cat has chronic itching or respiratory signs, allergy testing may be part of the workup. ### Will my cat need a bone marrow test? Usually not. Bone marrow sampling is reserved for cats with very high or persistent counts, immature cells on the blood smear, or signs of bone marrow disease [5]. Most cats are diagnosed with simpler tests first. ### Can steroids change my cat's eosinophil count? Yes. Corticosteroids commonly lower the eosinophil count, which can hide the real problem. Tell your veterinarian about any steroid your cat has received, including topical or injectable forms. ### How often should eosinophils be rechecked? It depends on the cause and how high the count is. Your veterinarian will set the interval. Mild, unexplained elevations are often rechecked in a few weeks, while serious diagnoses may need more frequent monitoring. ## Related Articles - [Eosinophilic Granuloma In Cats: Comprehensive Veterinary Reference Guide](/knowledge/veterinary-medicine/preventive-care/eosinophilic-granuloma-in-cats) - [Eosinophilic Granuloma Complex Cats: Comprehensive Veterinary Reference Guide](/knowledge/veterinary-medicine/preventive-care/eosinophilic-granuloma-complex-cats) - [Feline Hypereosinophilic Syndrome: Diagnosis and Management](/knowledge/veterinary-medicine/clinical-methods/feline-hypereosinophilic-syndrome-diagnosis-management) - [Leukogram Patterns in Dogs and Cats: A Diagnostic Guide](/knowledge/veterinary-medicine/clinical-pathology/leukogram-patterns-dogs-cats) - [Cat Asthma: Signs, Diagnosis, and Long-Term Veterinary Management](/knowledge/veterinary-medicine/symptom-guides/cat-asthma) - [Thyroid Testing in Cats: Diagnosing Hyperthyroidism and T4 Fluctuations](/knowledge/veterinary-medicine/at-home-diagnostics/thyroid-testing-in-cats-diagnosing-hyperthyroidism-and-t4-fluctuations) ## References 1. [White Blood Cells of Cats - Cat Owners](https://www.merckvetmanual.com/cat-owners/blood-disorders-of-cats/white-blood-cells-of-cats). Merck Veterinary Manual. 2. [Peripheral blood eosinophilia in dogs and cats: A cross-sectional study on prevalence, seasonality, and endoparasite infections in a referral hospital population](https://pubmed.ncbi.nlm.nih.gov/42156203/). Research in veterinary science, 2026. 3. [Congestive heart failure secondary to hypereosinophilic syndrome with cardiac involvement in a cat](https://pubmed.ncbi.nlm.nih.gov/42263443/). Journal of veterinary cardiology : the official journal of the European Society of Veterinary Cardiology, 2026. 4. [Feline paraneoplastic hypereosinophilic syndrome associated with lymphocytic intestinal lymphoma](https://pubmed.ncbi.nlm.nih.gov/42422439/). Brazilian journal of veterinary medicine, 2026. 5. [Suspected chronic eosinophilic leukemia in a cat treated with toceranib](https://pubmed.ncbi.nlm.nih.gov/42504294/). JFMS open reports, 2026. 6. [Secretoglobin Felis Domesticus 1 (Fel d 1) specific immunoglobulin E in diagnosis of patients with respiratory allergy to cats](https://pubmed.ncbi.nlm.nih.gov/41876634/). Scientific reports, 2026. ## Further Reading - [Merck Veterinary Manual: Cat Owners](https://www.merckvetmanual.com/cat-owners) - [Cornell Feline Health Center: Health Information](https://www.vet.cornell.edu/departments-centers-and-institutes/cornell-feline-health-center/health-information) - [American Veterinary Medical Association: Pet Care](https://www.avma.org/resources-tools/pet-owners/petcare)